Objectives: The effects of pCa on force-velocity relation and on the compliance of the series elastic element of cardiac muscle will be studied. A comparison of ATPase, force and velocity activities under the influence of pCa, pH and ionic strength will be made in order to determine the interdependence of these parameters of cardiac activation. The mechanisms by which various cardioactive drugs influence SR ATPase activity will be studied by analyzing their effects on SR phosphorylation, dephosphorylation and equilibrium with ADP. The drugs which will be studied include quinidine, propranolol and chlorpromazine.